Here is the basic principle of operation of a geothermal power plant. People have geothermal water from the earliest times as they used to cook using the hot water that flowed from the earth's surface. The Romans were actually using to treat eye and skin diseases, while the Pompeii were using to heat buildings. Native Americans were also using geothermal energy for cooking and medicine. In present days, geothermal power plants are taking it to the next step and use geothermal energy to generate electricity. The first geothermal power plant went into operation at The Geysers in 1921. It produced 250KW of electricity.

Geothermal power plants use heat, steam, or hot water from geothermal reservoirs as they provide the force that spins the turbine generators to produce electricity. There has got to be an external force that has to make the turbine generators spin and that's exactly the purpsoe of hot water in this case, or heat, or steam. Geothermal energy is a renewable type of energy because once the geothermal water is used, it's returned down by re-injection that puts it well into the reservoir to be reheated, to maintain pressure, and to sustain the reservoir.

There are three types of geothermal power plants. These power plants are differentiated depending on the temperatures and pressures of a reservoir, so here is the list:

-Dry steam reservoir - the largest Dry steam reservoir type of geothermal power plant is The Geysers (http://www.geysers.com/). It's in the Mayacamas Mountains, located 72 miles north of San Francisco, where naturally occurring steam field reservoirs below the earth's surface are being harnessed by Calpine to make clean, green, renewable energy for homes and businesses across Northern California. The Geysers, comprising 30 square miles along the Sonoma and Lake County border, is the largest complex of geothermal power plants in the world.

-hot water reservoir - geothermal reservoir that produces mostly hot water. The power plant that utilizes it is called a "flash" power plant.

-reservoir with temperatures between 250 and 360 degrees F - Such a reservoir doesn't have water hot enough to flash enough steam, but a binary power plant can still use it to produce electricity.

Pictures of geothermal power plants can be easily found on the Internet using the Image feature of the Google search engine.

Another serious advantage of geothermal energy is that it's available 24 hours a day, 7 days a week. The US Department of Energy has estimated that geothermal power plants have average availabilities of 90% or higher, compared to about 75% for coal plants. On top of all that, this power is homegrown, which means the United States of America is less dependent on foreign oil.

But the list of advantages of geothermal energy doesn't end here. It's a renewable energy source, because it uses the almost unlimited amount of heat generated by the Earth's core. As humanity is facing serious environmental problems like pollution and global warming, it's of vital importance that we switch to renewable energy sources, as soon as these become sufficiently sophisticated and cheaper. New technologies are to be introduced, reducing energy and raw material consumption (cellular phones, information storage and transfer on/through electronic media), rejection of heavy infrastructure, transport avoiding fossil fuels (bicycles, electric and other non-polluting cars, airships replacing airplanes, etc).

Geothermal technologies offer many environmental advantages compared to traditional power generation. As mentioned, the biggest one is that emissions are low, with the only excess being steam which is emitted by geothermal flash plants. No air emissions or liquids are discharged by binary geothermal plants, which the US Department of Energy plans to make the dominant technology in the near future. Salts and dissolved minerals contained in geothermal fluids are usually re-injected with excess water back into the reservoir at a depth well below groundwater aquifers. This recycles the geothermal water and replenishes the reservoir.

Of course there are advantages and disadvantages of geothermal energy. As a disadvantage can be taken the fact that it requires a substantial initial investment to build a geothermal power plant. Another disadvantage is that it can only be build on certain places with special characteristics. Search the web for more advantages disadvantages of geothermal energy.